Proper cardiac function relies on the synchronized contractions of the heart at regular intervals. When normal cardiac rhythm is initiated at the sinoatrial node, the heart is said to be in sinus rhythm. However, when the heart experiences irregularities in its coordinated contraction, due to electrophysiologic disturbances caused by a disease process or from an electrical disturbance, the heart is denoted to be arrhythmic. The resulting cardiac arrhythmia impairs cardiac efficiency and can be a potential life threatening event.
Cardiac arrhythmias occurring in the atria of the heart, for example, are called supra-ventricular tachyarrhythmias (SVTs). SVTs take many forms, including atrial fibrillation and atrial flutter. Both conditions are characterized by rapid, uncoordinated contractions of the atria. Besides being hemodynamically inefficient, the rapid contractions of the atria can also adversely effect the ventricular rate. This occurs when the aberrant contractile impulse in the atria are transmitted to the ventricles. It is then possible for the aberrant atrial signals to induce ventricular tachyarrhythmias.
Cardiac arrhythmias occurring in the ventricular region of the heart, by way of further example, are called ventricular tachyarrhythmias. Ventricular tachycardia (VTs), for example, are conditions denoted by a rapid heart beat, 150 to 250 beats per minute, that has its origin in some abnormal location with the ventricular myocardium. The abnormal location typically results from damage to the ventricular myocardium from a myocardial infarction. Ventricular tachycardia can quickly degenerate into ventricular fibrillation (VF). Ventricular fibrillation is a condition denoted by extremely rapid, non synchronous contractions of the ventricles. This condition is fatal unless the heart is returned to sinus rhythm within a few minutes.
Implantable cardioverter/defibrillators (ICDs) have been used as an effective treatment for patients with serious ventricular tachyarrhythmias. ICDs are able to recognize and treat tachyarrhythmias with a variety of tiered therapies. These tiered therapies range from providing anti-tachycardia pacing or cardioversion energy for treating ventricular tachycardia to defibrillation energy for treating ventricular fibrillation. To effectively deliver these treatments, the ICD must first identify the type of tachyarrhythmia that is occurring, after which appropriate therapy is provided to the heart. In order to apply the proper therapy in responding to an episode of tachyarrhythmia, the ICD may compare sensed cardiac signals to a previously stored normal sinus rhythm (NSR) signal waveform. It is appreciated that the stored NSR signal waveform must accurately characterize a patient's true normal sinus rhythm in order to properly identify potentially fatal deviations from normal cardiac activity.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading the present specification, there is a need in the art for reliably and accurately characterizing a patient's normal cardiac rhythm. There exists a further need for such an approach that is adaptive and accommodates changes in the patient's normal cardiac rhythm over time. The present invention fulfills these and other needs.